Change "user' on mail

[Donna Shelley]

Hi all, hope you're all well.  I've got this little "thing" about my mail.  When I set up my 

mail on my mac all went fine until there was a message to click on who the 'user' of the mail would be.  There were choices of my son, Guy Shelley Jr. (who passed away in 2015), then me, Donna Shelley...maybe more...but I was in a hurry and inadvertently clicked my sons name Guy Jr by mistake.   Oops....Now every mail is going to him...as if he is the main user.

He's not even here anymore, and I don't know how to change up the main user choice.  My name is on the account....but mail thinks that Jr. is the recipient. 

How can I change that?

Thanks as always, Donna Shelley